DESCRIPTION (abstract, from application): Flow cytometers provide invaluable information for the fields of developmental biology, genetics, hematology, immunology, oncology, microbiology, and parasitology. Multiparameter analysis greatly enhances the capability of flow cytometers, and reducing the size, power, and cost of the laser excitation source improves their commercial viability. Unfortunately, the use of multiple, photomultiplier tubes (PMT's) substantially increases the cost. Furthermore, PMTs are not readily compatible with the emerging microscale technology that is now being applied to flow and scanning cytometry. Cytometry needs a low-cost array of highly sensitive, real-time photodetectors that are red-sensitive and compatible with microscale technology. Geiger-mode, micro-avalanche photodiode (uAPD) arrays are among the most sensitive optical detectors available today. In Geiger-mode, they provide noiseless amplification while counting individual photons. Although photon counting achieves the best sensitivity for a photodector, it frequently limits the effective dynamic range of the device. We propose to conduct the research and development necessary to implement photon counting, Geiger-mode uAPD arrays for flow cytometry. We will explore designs to extend the dynamic range of these photon-counting devices. This will result in a low cost, high efficiency alternative to PMTs, which will be compatible with microscale flow cytometry systems and other microanalysis systems. PROPOSED COMMERCIAL APPLICATION: The development of an inexpensive replacement for photon-counting PMTs would have many applications. High performance, Geiger-mode APDs are already a commercial product, and inexpensive replacement for a PMT would find widespread commercial appeal. These uAPD arrays are compatible with the growing microanalysis technology, which includes microscale flow cytometers, DNA sequencing chips for criminal identification, analysis systems for detecting and identifying biological agents, "point-of-care" systems, and high- throughput screening system for the pharmaceutical industry.